1. Field of the Invention
The invention relates generally to semiconductor fabrication and more specifically to process control during chemical mechanical planarization (CMP) wafer processing.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform Chemical Mechanical Planarization (CMP) operations, including removal of the excessive material, buffing and post-CMP wafer cleaning and drying. Typically, integrated circuit devices are manufactured in the form of multi-level structures. At the substrate level, transistor devices having p-type and n-type doped regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive features are insulated by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially impossible due to variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation of for poly-metal insulation.
Typically CMP systems implement a rotary table, belt, orbital or brush operation in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of wafer. The pad itself is typically made of polyurethane material or other suitable material and may be backed by a rigid table, supporting belt, for example a stainless steel belt. In operation a slurry liquid is applied to and spread across the surface of the polishing pad or belt. As the belt or pad covered with slurry rotates, a wafer is lowered to the surface of the pad and is planarized.
The desired result of successful CMP operations is a uniform planar surface remaining on the processed wafer. Typically the removal rate of films on the wafer is carefully tracked or monitored. Various attempts have been made to control the operation of chemical mechanical planarization systems in order to provide uniform removal rates. One common attempt is to control down force applied by the wafer carrier or other workpiece holding device, delivering variable pressure to an abrasive polishing surface. Unfortunately, down force variation can lead to local degradation of so-called dishing and erosion performance at the sections of the wafer where high compensative down force was applied. Excessive down force may cause additional quality issues like film delamination, scratching, or inter-grain boundary damage. The focus on uniformity of removal rates may sometimes be misguided for current applications. That is, from the end user's perspective, it is desired to have a uniform post-CMP layer on the surface of the semiconductor wafer, which is not necessarily the result of uniform removal rate. For example, if the surface of the wafer prior to planarization is not uniformly thick, the non-uniformities are maintained when a uniform removal rate is applied to the processed wafer. A uniform removal rate applied to substrate with greater edge thickness will result in a wafer with a lower center thickness, a condition similar to the wafer prior to the planarization application. Additionally, in the example related above, over-polishing of the center of the wafer can result in lost die and lower wafer yield.
During the CMP operation there are many opportunities for measuring device features on wafers. Many of the features can be determined by capturing a signal indicating the feature. As features continue to decrease in size, especially the thickness of films employed in the manufacture of semiconductors, the signals that are indicative of the feature become undetectable in certain situations. Inductive sensors may be used for displacement, proximity and film thickness measurements. The sensors rely on the induction of current in a sample by the fluctuating electromagnetic field of a test coil proximate to the object being measured. Fluctuating electromagnetic fields are created as a result of passing an alternating current through the coil. The fluctuating electromagnetic fields induce eddy currents which generate their own fields, superimposing with the primary field and change the coils inductance. Feedback from sensors such as inductive sensors during planarization can allow for real-time monitoring and correction if needed during the CMP operation.
In view of the foregoing, a technique for controlling the composition and amount of fluids supplied differentially on a polishing pad during a planarization operation is needed to more accurately influence the removal rate variation over a wafer surface.